1. Field of the Invention
This invention relates to an electroluminescent display (EL display) device comprising electroluminescent elements (EL elements) each having electroluminescent layers (EL layers) which generate light with application of an electric field thereto.
2. Description of the Related Art
Conventionally, a cathode ray tube (CRT) or a liquid crystal element is widely used for a display device which is capable of displaying multiple colors in response to electric signals. Further, there have been developed an EL display device comprised of a plurality of completely solid-state EL elements which are capable of emitting very bright light by electroluminescence.
When classified according to the material of a light-emitting layer of an EL element which emits light with application of an electric current, there are two groups of EL display devices: a group using an EL element having an inorganic light-emitting layer and a group using one having an organic light-emitting layer. In recent years, much attention has been focused on the EL elements having an organic light-emitting layer.
An organic EL element is comprised of a pair of electrodes and laminated layers of organic fluorescent materials, such as a hole transport layer, a light-emitting layer, and an electron injection layer, which are formed between the electrodes. FIG. 1 shows an example of an organic EL display device of X-Y matrix type. This device includes EL elements 10 each having a two-layered structure. The EL element 10 has a plurality of transparent electrode stripes (anodes) 2 formed of indium tin oxide (ITO) or the like on a transparent glass substrate 1 on which a hole transport layer 3 and an organic light-emitting layer 4 are deposited in the mentioned order, on which a plurality of back electrodes (cathodes) 6 are formed and arranged crosswise to the transparent electrodes 2. The EL elements at intersections of the electrodes 2 and 6 form a matrix of light-emitting points corresponding to pixels. In addition to the illustrated type having the two-layered structure of the hole transport layer 3 and the organic light-emitting layer 4, there are other types of organic EL display device, such as one using EL elements of a three-layered structure type having a layer of an organic electron transport layer additionally provided between the organic light-emitting layer 4 and the back electrodes 6, and another using multiple-layered EL elements formed of still more layers. It should be noted that the hole transport layer, the organic light-emitting layer, the electron transport layer, and the like are also referred to as "organic electroluminescence functional layers".
The organic light-emitting layer 4 of the organic EL element 10 is a layer including a fluorescent material such as a coumarin compound. When application of a voltage across the electrodes 2 and 6, the organic hole transport layer 3 has the functions of facilitating injection of holes from the positive electrodes and blocking electrons, while the organic electron transport layer has the function of facilitating injection of electrons from the negative electrode. In the organic EL element 10, an electron and a hole injected from the corresponding electrodes are recombined to generate an exciton. The exciton emits light upon deactivation thereof, and the light is sent out via the transparent electrode and the glass substrate.
However, if the last layered cathode formed of Aluminum Al, for example, has a pinhole 7 as shown in FIG. 1, then moisture, oxygen or the like permeates into the organic layers of the EL element 10 through the pinhole 7 to cause an increase in the electrical resistance at an interface between the cathode 6 and the light-emitting layer 4 (or separation of the cathode 6 from the layer 4), or cause deterioration of the organic electroluminescence functional layers, whereby an electroluminescence-disabled region, i.e., a dark spot 8 is produced.
Further, there may be a little possibility of dust adhesion 15 on the organic EL display as shown in FIG. 1. Since the organic electroluminescence functional layers have a very small thickness of a submicron order, a short circuit due to the dust 15 is liable to occur between the transparent electrode (anode) 2 and the back electrode (cathode) 6 as shown in FIG. 1. The conventional countermeasure for preventing occurrence of short circuits are washing and smoothing of the surface of the substrate. However, it is impossible to perfectly clean the substrate surface, there still remains possibility of occurrence of short circuits.